1. Field of the Invention
The present invention relates generally to a system for the detection and quantification of lattice defects in semiconductor materials. The invention, more specifically, concerns a method and apparatus for nondestructive determination of lattice defects by the technique of positron lifetime spectroscopy using an array of detectors.
2. Description of Related Art
The technique of positron annihilation is known in the art as a method for nondestructive testing of defects in crystalline structures, as well as for determining metal fatigue. In the basic method a positron is emitted from a radioactive material (.sup.22 Na for example) placed proximate to the specimen material. At approximately the same time, a nuclear gamma ray is emitted, signalling the birth of the positron. This nuclear gamma ray (or "start" ray) has a unique energy of approximately 1.28 MeV. When the positron enters the specimen material, such as a semiconductor wafer, it quickly loses its energy, reaches thermal equilibrium, and seeks out an electron in the material with which it interacts (annihilates). The interaction of positive and negative charges results in complete annihilation of both the positron and electron, and further causes the emission of two (or, rarely, three) gamma rays each having a unique characteristic energy of approximately 0.51 MeV. As a result, the end of the lifetime of an individual positron is signalled by the radiation of a unique energy. Once the time difference between the emission of the 1.28 MeV "start" gamma ray and the 0.51 MeV "stop" gamma rays is known, the time duration of existence of the positron in the material may be determined.
The lifetime of the positron in a material is instructive on the existence of defects in that material because it has been found that the lifetime of positrons is longer in materials containing defects such as vacancies and microvoids. The theory behind the increased positron lifetime is that the positrons become trapped in the voids, thus staying alive for a longer time due to a reduction in the number of available electrons inside the defect. A precise measurement of changes in the lifetime of positrons may reveal much information about the defects present in the sample material.
The foregoing discussion of the positron annihilation technique is by way of background only. Further details may be had by reference to U.S. Pat. No. 3,593,025, entitled "Detecting Defects by Distribution of Positron Lifetimes in Crystalline Materials," issued to Grosskreutz on Jul. 13, 1971, the disclosure of which is herein incorporated by reference. In addition, more information on the theory of positron annihilation spectroscopy may be had by reference to Positron Solid-State Physics, edited by W. Brandt and A. Dupasquier, North-Holland, Amsterdam, 1983.
The positron annihilation technique described above is a standard research laboratory technique. It has not yet, however, evolved into an "industrial tool" for quality control of defects in semiconductor materials, for two basic reasons: (1) the data collection and analysis takes too long to be suitable for the quality control of defects in an industrial environment, and (2) the technique has largely been geared toward the study of metallic samples, which are relatively easier to control and understand as compared to semiconductor wafers.
With regard to semiconductor materials specifically, basic laboratory experiments have demonstrated the potential of the positron annihilation technique for the investigation of defects such as vacancies and microvoids. Such information is not, in fact, known to be available from other nondestructive techniques.
From a practical standpoint, knowledge of the purity of a semiconductor material is critical, in that the presence or absence of defects in a semiconductor wafer determines whether a device made from that wafer will meet desired specifications. Due to inherent problems in the growth process of silicon wafers, significant differences may exist in the types and concentrations of defects throughout different wafers from the same ingot, or even throughout a single wafer. Variations in defects within a single wafer could cause electronic chips made from the same wafer to perform very differently. In other words, whereas one device may perform beautifully, another device made from the same wafer may totally fail. Thus, random "spot" testing is not an effective method for accurately predicting the performance of an entire semiconductor wafer. Only knowledge of the vacancy contents (or presence of defects) across an entire wafer will permit industry to accurately predict whether to use an entire wafer, or selected portions, for device fabrication, or whether to discard a defective wafer altogether. Such reliable prediction will greatly reduce the expense associated with defective semiconductor devices and also with wasting acceptable portions of otherwise defective wafers.
In U.S. Pat. No. 4,897,549, Zerda et al. discuss a method of measuring pore diameters by positron decay. In the method described, a radioactive source is sandwiched between two specimens of the sample material and a scintillation counter is disposed on either side of the specimen material. A positron is emitted from the radioactive source and enters the material. Once it enters the material, the positron decays into the characteristic gamma rays described above. Phototubes are provided to detect the characteristic energy emission. Constant fraction differential discriminators are coupled to the phototubes to pass signals corresponding to the prompt gamma ray emission, which in turn pass signals to a time-to-amplitude converter and then to a multichannel analyzer. Each spectrum is measured for 1 to 8 hours to accumulate a sufficient number of pulses. Further discussion of this technique may be had by reference to S. C. Sharma et al., "Depth and Radial Profiles of Defects in Czochralski-grown Silicon," 61 Appl. Phys. Lett. pp. 1939-1941 (Oct. 19, 1992).
Using the positron lifetime measurements progressively made across an entire wafer, it is theoretically possible to map lattice defects across the entire wafer. However, the substantial time involved is prohibitive. In the typical method of positron-annihilation testing (such as that described in Zerda et al. and Sharma et al.), the measurement of a lifetime spectrum provides defect information over a spot size of only about 2 to 3 mm.sup.2 on a wafer. Each spectrum in turn requires 1 to 8 hours of measurement time. Then, to map the entire surface of a typical 6 to 8 inch semiconductor wafer, the positron source must be manually displaced. Thus, it could take one to two weeks using conventional positron annihilation methods to scan a single semiconductor wafer. This time period would be prohibitively long in the industrial environment, where it is desirable to scan large numbers of wafers almost routinely.
Thus, a need exists for a technique of nondestructively yet rapidly determining semiconductor wafer defects in an industrial environment.